RFC 3385 (rfc3385) - Page 2 of 23
Internet Protocol Small Computer System Interface (iSCSI) Cyclic Redundancy Check (CRC)/Checksum Considerations
Alternative Format: Original Text Document
RFC 3385 iSCSI CRC Considerations September 2002 developers for many years. However, algorithms and software for effective implementations of CRC are now also widely available [Williams]. The probability of undetected errors depends on the polynomial selected to generate the code, the error distribution (error model), and the data length. 2. Error Models and Goals We will analyze the code behavior under two conditions: - noisy channel - burst errors with an average length of n bits - low noise channel - independent single bit errors Burst errors are the prevalent natural phenomenon on communication lines and recording media. The numbers quoted for them revolve around the BER (bit error rate). However, those numbers are frequently nothing more than a reflection of the Burst Error Rate multiplied by the average burst length. In field engineering tests, three numbers are usually quoted together -- BER, error-free-seconds and severely-error-seconds; this illustrates our point. Even beyond communication and recording media, the effects of errors will be bursty. An example of this is a memory error that will affect more than a single bit and the total effect will not be very different from the communication error, or software errors that occur while manipulating packets will have a burst effect. Software errors also result in burst errors. In addition, serial internal interconnects will make this type of error the most common within machines as well. We also analyze the effects of single independent bit errors, since these may be caused by certain defects. On burst, we assume an average burst error duration of bd, which at a given transmission rate s, will result in an average burst of a = bd*s bits. (E.g., an average burst duration of 3 ns at 1Gbs gives an average burst of 3 bits.) For the burst error rate, we will take 10^-10. The numbers quoted for BER on wired communication channels are between 10^-10 to 10^-12 and we consider the BER as burst-error-rate*average-burst-length. Nevertheless, please keep in mind that if the channel includes wireless links, the error rates may be substantially higher. For independent single bit errors, we assume a 10^-11 error rate. Sheinwald, et. al. Informational
